An X-ray tube is a vacuum tube that produces X-rays. The X-ray tube includes a cathode for emitting electrons into the vacuum and anode to collect the electrons. A high voltage power source is connected across the cathode and anode to accelerate the electrons. Some applications require very high-resolution images and require X-ray tubes that can generate very small focal spot sizes.
One type of cathode includes a tungsten filament that is helically wound in a spiral, similar to a light bulb filament. The problem with the wound filament is that the electrons are emitted from surfaces that are not perpendicular to the accelerating electrical fields. This makes it very difficult to focus the electrons into a compact spot on the x-ray target.